1. Field of the Invention
The invention relates to machines for lapping optical surfaces, and particularly to planetary or continuous polishing machines, and most particularly the control of the shaping or lapping surface of such machines.
2. Background Art
Planetary grinding or polishing machines (generally called lapping machines herein) are well-known tools for manufacturing plano optical surfaces. A typical machine has a relatively large, circular, horizontal, optically flat lapping surface which is driven about an axis. One or more circular glass blanks, or work pieces, to be shaped or polished are disposed on the lapping surface near its perimeter, with the blank surfaces to be polished facing the lapping surface. The glass blanks have diameters smaller than the radius of the lapping surface and typically are pressed against the lapping surface by their own weight. Each blank is constrained from being carried along by the lapping surface by a stationary apparatus which additionally rotates the blank about its own axis while the lapping surface passes beneath it. A slurry of polishing compound, typically containing cerium oxide, is flowed between the blank and the lapping surface while both are turning about their respective axes, and the relative motion causes the blank to become shaped and polished uniformly in conformance with the lapping surface.
In some applications, the planarity of the blank must first be established. More aggressive slurries may be needed at the beginning to grind the work piece flat before final polishing can occur. Grinding and polishing as used herein can be thought of as opposite ends of a spectrum of aggression in material removal from a blank by the lapping procedure described above.
During lapping of the blank or blanks, the lapping surface may be reshaped by the polishing compound and may require conditioning to maintain its planarity. Typically, a large, heavy glass body, or "conditioner," is polished simultaneously with the work pieces, and is periodically radially positioned to keep the lapping surface flat.
Planetary polishing technology has also been applied to the grinding and polishing of high quality spherical surfaces. The lapping surface, or "table," is provided as a spherical segment which can be either concave or convex as needed, and blanks are then finished as either convex or concave complementary surfaces, respectively.
To produce a work piece having an optical surface with a desired radius of curvature, it is essential that the radius of curvature of the lapping surface be maintained throughout the operation. Typically, a conditioner body is used, as in the plano case, to continuously shape the lapping surface which otherwise becomes progressively erroneous in shape through wear from lapping of the work piece. The conditioner can be moved radially inward or outward on the lapping surface to correct and maintain the radius of curvature, provided that the actual shape of the table and its deviation from ideal are known.
The curvature of the table and requirement for shape correction has heretofore been inferred in at least two ways, both of which require that lapping be interrupted. This reduces the runtime efficiency of the machine. First, the work piece itself can be removed and measured directly. Discrepancies in the table can be inferred from the measurements, and the table can be reshaped by running the table with the conditioner repositioned before the work proceeds further. This is very labor-intensive, time-consuming, hazardous to the work piece, imprecise, and impractical for large glass blanks. Another common approach is to include a small "monitor" blank of glass on the lapping surface in one of the regular work stations, where blanks may be located for lapping during operation of the machine, or in a monitor station. The monitor is removed and checked periodically for surface error, from which the surface error of the table is inferred. This control method has at least three drawbacks. First, the monitor may occupy work space and thus reduces productivity of the machine. Second, the monitor is typically much smaller than the work piece, as when planetary polishing is scaled up for production of large precision optical elements such as mirrors or lenses having diameters larger than, for example, one meter. For such large elements, measurement and control of the radius of curvature of the grinding or polishing surface, and hence of the work piece, becomes progressively less accurate, since the accuracy of measurement of curvature of the monitor is proportional to the square of its diameter. The use of a large (full-scale) monitor to improve measurement accuracy would further reduce the amount of polishing table surface available for production. Third, the time delay required for removing and evaluating the monitor off-line can cause loss of control of the table curvature as lapping continues during evaluation. If lapping is suspended, however, productivity is affected.